Dr. Brent Tetri of Saint Louis University, an expert on non-alcoholic fatty liver disease, says a diet high in fat and sugar could cause serious damage to the liver.

Tetri says U.S. physicians are starting to see children and teenagers with cirrhosis, a serious liver disease once seen mostly in adults with a history of alcohol abuse or hepatitis C.

Tetri says fast food doesn't causes liver damage -- the harm comes from too many calories and too much fat and sugar. To undo a high-fat, high-sugar diet, Tetri suggests to:

-- Have only one fast-food meal a week.

-- Even when eating fast food, eat as healthfully as possible. Try the burger without mayonnaise and cheese, and avoid fries and sugary soft drinks. Try a grilled chicken sandwich, a salad with a lower-fat dressing and bottled water.

-- Exercise at least three times a week.

-- Ask your doctor to do a blood test to check liver enzymes -- a key measure of the health of your liver.